ci: align pnpm tooling with v11 migration

- Add pnpm 11.5.1 to .mise.toml and .tool-versions
- Upgrade pnpm/action-setup from v4 to v6 to fix
  'self-installer exits with code 1' under pnpm 11

Author: EugeniyKiyashko

.github/actions/setup-env/action.yml

@@ -15,7 +15,7 @@ runs:
       with:
         node-version-file: ${{ inputs.path }}/.node-version
 
-    - uses: pnpm/action-setup@v4
+    - uses: pnpm/action-setup@v6
       with:
         run_install: false
         package_json_file: ${{ inputs.path }}/package.json

.mise.toml

@@ -1,5 +1,6 @@
 [tools]
 node = "24.15.0"
+pnpm = "11.5.1"
 
 [hooks]
 enter = "mise install"

.tool-versions

@@ -1 +1,2 @@
 nodejs 24.15.0
+pnpm 11.5.1